1. Field of the Invention
The present invention relates generally to a hydraulic tilt system for a marine propulsion device, and more particularly to a hydraulic tilt system that has a relief valve for relieving pressure that is applied to a working fluid.
2. Description of Related Art
Relatively small watercrafts can be provided with a marine propulsion device that has a drive unit mounted on a transom of the watercraft. The drive unit carries a propelling unit such as, for example, a propeller that propels the watercraft. A typical marine propulsion device is an outboard motor.
The outboard motor typically has a bracket assembly to support the drive unit on an associated watercraft. The bracket assembly includes a swivel bracket carrying the drive unit for pivotal movement about a steering axis, and a clamping bracket supporting the swivel bracket for pivotal movement about a tilt axis that lies generally normal to the steering axis.
The outboard motor can be provided with a hydraulic tilt system that tilts the drive unit together with the swivel bracket relative to the clamping bracket and, thus, relative to the watercraft. The hydraulic tilt system can incorporate a tilt cylinder extending generally vertically. A tilt piston is slidably disposed within the tilt cylinder and divides an inner cavity of the tilt cylinder into first and second chambers. A piston rod extends from the piston through the second chamber and outwardly beyond one end of the tilt cylinder. The piston rod can be coupled with the swivel bracket or the clamping bracket, while the other end of the tilt cylinder can be coupled with the rest of the clamping bracket or the swivel bracket.
A fluid pump pressurizes a working fluid that is selectively delivered to the first chamber and the second chamber such that the piston rod extends or retracts relative to the tilt cylinder, respectively. With the extending or retracting movement of the piston rod, the swivel bracket together with the drive unit is tilted up or down, respectively.
The working fluid in the first or second chamber can have excessive (i.e., unnecessarily high) pressure. For instance, the fluid pump can continuously pressurize the working fluid even after the piston has reached the limit of its stroke at either end of the tilt cylinder. Under the continuous pressurization of the working fluid, the first or second chamber can build higher pressure than is necessary. In order to relieve the pressure built in the first or second chambers, relief valves, which allow the fluid to move toward, for example, a fluid reservoir, can be provided.
Such relief valves are well known and, for example, U.S. Pat. Nos. 4,778,414, 4,813,896, 4,909,766, 4,990,111 and 5,984,741 disclose the relief valves.
In general, the drive unit with the swivel bracket can move in a range between a fully lowered position and a fully raised position. The range is conveniently divided into a trim adjustment range and a tilt range. The trim adjustment range covers a portion of the range from the fully lowered position to a certain position. Thrust force of the propeller can slightly lift up the bow of the watercraft when the drive unit is in the trim adjustment range and the watercraft can relatively easily gather speeds for high speed running. The tilt range, in turn, covers the rest of the range from the certain position to the fully raised position. The entire drive unit can be raised out of a body of water with the watercraft resting on the surface of the water body when the drive unit is placed at a relatively higher position in the tilt range. The propeller also is out of the body of water under this condition.
The drive unit typically is continuously raised from the trim range to the tilt range. Normally, the operator stops the operation of the hydraulic tilt system at a suitable position of the drive unit in the trim range when moving. Thus, the drive unit normally does not raise into the tilt range without an intentional operation of the tilt system by the watercraft's operator.
Sometimes, however, the operator does not notice that the drive unit is moving into the tilt range and the operator can accelerate the engine speed without stopping the operation of the tilt system. Under this condition, relatively large pressures can occur in the first or second chamber as a result of the weight of the outboard motor and the thrust force of the propeller. If the pressure is larger than a preset pressure of the relief valve, the relief valve can open to relieve the pressure and the drive unit lowers to the trim range.
The preset pressure, however, varies in accordance with the properties of the associated watercrafts and the outboard motors. That is, the preset pressure depends on a size of the watercrafts and a horsepower of the outboard motors, for example. Various types of relief valves thus are necessary for being adapted to those watercrafts and outboard motors. However, preparing those relief valves increases cost and also adds the troublesome management of the relief valves.